The goal of the proposed study is to investigate the cognitive effects of physical activity in two groups - young and older adults - aged 25-40 and 50-65. The great majority of controlled exercise studies in humans have been restricted to elderly individuals, typically age 65 and up. However, earlier intervention may be more beneficial for preventing or slowing age-related changes. Exercise studies in older adults have indicated that enhancing aerobic capacity has a beneficial effect on cognition, particularly executive control processes. However, the specific cognitive domains that are benefited may differ in younger and older individuals, since the most responsive domains in elders are those affected by aging. In the proposed 6-month randomized intervention study, 260 sedentary but otherwise healthy and cognitively intact individuals in these two age groups will be randomized into two treatment arms, aerobic exercise and stretching / toning. The potential effect of exercise on multiple cognitive domains will be examined with standard cognitive evaluations and computerized cognitive tasks. In addition, 3 levels of neuroimaging studies: structural MRI (for gray matter density), resting CBF (arterial spin labeling) and cognitive activation fMRI studies will be conducted at baseline and 6 months. We hypothesize that: 1) in comparison to the stretching / toning condition, aerobic exercise will produce improved performance on measures of cognition; 2) cognitive improvement will be noted regardless of age, although it will be greater in the older group; 3) in both younger and older participants in the aerobic condition, improvement will be more notable for tasks that tap executive control processes, such as set switching and working memory, compared to other cognitive domains; 4) improved in task performance will be accompanied by increased gray matter density, and increased efficiency of brain networks measured in fMRI studies. The proposed analysis of exercise effects at multiple levels will yield important insights onto the neural mechanisms underlying the beneficial effects of exercise. Because it may help prevent or delay cognitive aging, the possibility that exercise improves cognition in younger subjects has enormous potential significance for public health.